Convert Generator#_options to an object

Additionally, use `_.reject()`, instead of `_.filter` with a not.
Add Generator#parseOptions

`#parseOptions` will take the "private" #_args and add all the
generator's options to the #options object. Supports nopt's short hands
using the `alias` key of an option.

Depends upon getting the full arguments from the yo cli, not just the
remaining arguments.
https://github.com/yeoman/yo/blob/37ae024f0105311945c98aef7a369f450d71fe
9c/cli.js#L20
Test that `#argument`s will be updated

Since `#option()` rewrites `this.args`, make sure any `#arguments` will
update their values afterwards.
Remove unneeded feature
Use a getter for positional arguments

Since setting an option can change the positional arguments, use a
getter/setter.
#parseOptions() when calling #option()